1. Field of the Invention
The field of the invention is that of the decoding of digital data received in a decoder and, notably, of data transmitted in the presence of transmission noise.
More specifically, the invention relates to the decoding of data obtained by convolutional coding. In a known way, convolutional codes associate, with a source data element to be transmitted, at least one coded value obtained by the linear combination of this source data element with at least one of the preceding source data elements.
In a standard way, such codes are decoded by means of a maximum likelihood algorithm of the Viterbi algorithm type. This algorithm gives a binary estimation of each symbol coded at transmission. Thus, depending on the success of the decoding, this estimation proves to be either wholly correct or wholly false.
2. Description of the Prior Art
It has been seen that it is especially useful to weight the decisions taken by a maximum likelihood decision algorithm, notably when the convolutional code is concatenated with one or more other codes such as, for example, another code of a convolutional type. Indeed, this then enables the decoder of this concatenated code to be informed of the reliability of the estimation transmitted to it.
A weighted decision decoder then delivers an information element coded on n bits at output, the most significant bit being identical to that delivered by the standard decoder, and the remaining n-1 bits representing the reliability assigned to the decision.
It is known that the Viterbi algorithm, which shall be described in greater detail here below, is based on the determination of an optimal path in a trellis, by the systematic elimination of one path among two possible paths reaching each node of the trellis. For each node, therefore, two transition metrics are determined, these transition metrics representing the distance between the transition that is possible on each path and the value actually received by the decoder. These transition metrics enable the computation of the cumulated metrics, representing the cumulated noise on the path considered. A cumulated metric is thus an integral of the transition metrics. According to Viterbi's algorithm, only the path corresponding to the smallest cumulated metric is kept.
It can be clearly seen that the systematic elimination of a path introduces a risk of error, notably when the two computed metrics are close. By contrast, when one of the metrics is small and the other is great, the probability of estimation error is low. The first idea implemented to weight the Viterbi algorithm was therefore the association, with each node, of a weighting coefficient equal to the difference (in terms of absolute value) between the two cumulated metrics.
This mode of weighting, which is particularly simple, is not optimal in terms of results.
There also exists an improved decoding method known from the US patent document U.S. Pat. No. 4,811,346, granted on 07-03-1989. This is an improved decoding method with weighting of the symbols decoded by the Viterbi algorithm. According to this method, the weighting coefficient associated with a decision is revised periodically in taking account of the samples received subsequently.
This method gives results that are far more satisfactory than with the basic principle. By contrast, it proves to be extremely complex to implement. Indeed, it calls for the permanent memorizing of the two paths associated with each node as well as a large number of computations at each instant of reception. In practice, this method cannot be implanted in an integrated circuit for purposes of low-cost, industrial-scale distribution.
The invention is aimed notably at overcoming these different drawbacks of the prior art.